Yasuhiko Imai

Controlling charge-density-wave states in nano-thick crystals of 1T-TaS2

Two-dimensional crystals, especially graphene and transition metal dichalcogenides (TMDs), are attracting growing interests because they provide an ideal platform for novel and unconventional electronic band structures derived by thinning. The thinning may also affect collective phenomena of electrons in interacting electron systems and can lead to exotic states beyond the simple band picture. Here, we report the systematic control of charge-density-wave (CDW) transitions by changing thickness, cooling rate and gate voltage in nano-thick crystals of 1T-type tantalum disulfide (1T-TaS2). Particularly the clear cooling rate dependence, which has never been observed in bulk crystals, revealed the nearly-commensurate CDW state in nano-thick crystals is a super-cooled state. The present results demonstrate that, in the two-dimensional crystals with nanometer thickness, the first-order phase transitions are susceptible to various perturbations, suggestive of potential functions of electronic phase control.

Evidence of lattice tilt and slip in m-plane InGaN/GaN heterostructure

Using high-resolution microbeam x-ray diffraction and cross-section transmission electron microscopy, we investigated in-plane anisotropy resulting from epilayer lattice tilts in heteroepitaxial InGaN on a m-plane GaN substrate. The in-plane structure consists of two lattice tilts along the [112¯0] direction corresponding to (101¯0) and (01¯10) slip planes inclined at roughly 60° from the m-plane. Based on the Peierls-Nabarro model, we explain this structure by proposing a slip system via the {101¯0} prism plane with 〈112¯0〉-type slip directions.

High‐Angular‐Resolution Microbeam X‐Ray Diffraction with CCD Detector

We have introduced a CCD‐type two‐dimensional X‐ray detector for a microbeam X‐ray diffraction system using synchrotron radiation, so that we can measure local reciprocal space maps (RSM) of samples rapidly. A local RSM of a strain‐relaxed SiGe 004 grown on a Si (001) substrate was measured in higher‐angular‐resolution and faster than a conventional way. The measurement was achieved in 1 h 40 min. with the 2θ resolution of 80 μrad and the spatial resolution of 1.4(h)×0.5(v) μm2. The introduction of the CCD enabled us to measure RSMs at many points in a sample, that is, the distribution of strain fields and lattice tilts can be revealed in high‐angular‐ and high‐spatial‐resolution.

Impact of mechanical stress on ferroelectricity in (Hf0.5Zr0.5)O2 thin films

To investigate the impact of mechanical stress on their ferroelectric properties, polycrystalline (Hf0.5Zr0.5)O2 thin films were deposited on (111)Pt-coated SiO2, Si, and CaF2 substrates with thermal expansion coefficients of 0.47, 4.5, and 22 × 10−6/ °C, respectively. In-plane X-ray diffraction measurements revealed that the (Hf0.5Zr0.5)O2 thin films deposited on SiO2 and Si substrates were under in-plane tensile strain and that their volume fraction of monoclinic phase decreased as this strain increased. In contrast, films deposited on CaF2 substrates were under in-plane compressive strain, and their volume fraction of monoclinic phase was the largest among the three kinds of substrates. The maximum remanent polarization of 9.3 μC/cm2 was observed for Pt/(Hf0.5Zr0.5)O2/Pt/TiO2/SiO2, while ferroelectricity was barely observable for Pt/(Hf0.5Zr0.5)O2/Pt/TiO2/SiO2/CaF2. This result suggests that the in-plane tensile strain effectively enhanced the ferroelectricity of the (Hf0.5Zr0.5)O2 thin films.

Deteriorated Device Characteristics in 3D-LSI Caused by Distorted Silicon Lattice

Silicon-lattice distortion in the 50- μm-thick stacked large scale integrated circuit (LSI) chip over Cu-Sn μ-bumps was studied by synchrotron-assisted micro-X-ray diffraction. The top and bottom surfaces of the upper chip experienced 0.25% and 0.1% tensile strain (equivalent to 450 and 200 MPa of tensile stress), respectively. Si [004] plane showed a maximum tilt value of +0.45° and -0.25°, respectively, over the μ-bump and in the bump-space region. Raman spectroscopy revealed that upper stacked chip experienced ~ 1000 MPa of tensile stress and ~ -200 MPa of compressive stress, respectively, over the μ-bump and bump-space regions. Distorted Si-lattice in 3D-LSIs caused 4% and 12% change in ON-current characteristic for n- and p-MOSFET devices, respectively.

Characterization of domain structure in one-dimensional SrRuO3 nanostructure using synchrotron x-ray microdiffraction

SrRuO3 (SRO) thin films with a geometric shape of one-dimensional stripes can be epitaxially grown on a SrTiO3 (STO) substrate. Conventional X-ray reciprocal space map (RSM) measurements revealed that the stripes consist of multiple crystallographic domains. We performed synchrotron X-ray microdiffraction measurements to determine whether the single stripe of the SRO has a single crystallographic domain or not. Spacing between stripes is ~200 nm that is comparable to a beam size available for the microdiffraction. The synchrotron X-ray microdiffraction experiment was performed at BL13XU, SPring-8. RSMs of asymmetric diffractions around STO 204 reflection were measured by a broad-beam (200 × 200 µm2) and the sub-micro-beam (250(h) × 190(v) nm2). Both SRO 260 and 620 are seen in the RSM measured by the broad-beam due to the crystallographic twinning. On the other hand, only SRO 620 is observed in the RSM measured by the sub-micro-beam. The result shows the domain length of the single stripe SRO thin film is...

Large irreversible non-180° domain switching after poling treatment in Pb(Zr, Ti)O3 films

(11 1¯)/(111)-oriented rhombohedral Pb(Zr0.65Ti0.35)O3 films with different domain fractions were epitaxially grown on various single crystals. The volume fraction of (111)-polar-axis oriented domains in as-deposited films, Vpol.(as-depo.), was controlled by selecting a single crystal substrate with a different thermal expansion coefficient. Applying an electric field, referred to as “poling treatment”, resulted in irreversible non-180° domain switching from the (11 1¯)-oriented domain (non-polar-axis) to the (111)-oriented domain (polar-axis), which was observed by synchrotron X-ray diffraction. Remanent polarization (Pr) values were higher than those estimated using the proportional relationship with Vpol.(as-depo.). However, the experimental Pr values were in good agreement with the values estimated using the volume fraction of (111)-oriented domains after applying the poling treatment. In rhombohedral Pb(Zr0.65Ti0.35)O3 films, 30%−50% of the (11 1¯)-oriented domains switched irreversibly to (111)-orie...

Extended Polymorphism of Two-Dimensional Material

When controlling electronic properties of bulk materials, we usually assume that the basic crystal structure is fixed. However, in two-dimensional (2D) materials, atomic structure or polymorph is attracting growing interest as a controlling parameter to functionalize their properties. Various polymorphs can exist in transition metal dichalcogenides (TMDCs) from which 2D materials are generated, and polymorphism has drastic impacts on the electronic states. Here we report the discovery of an unprecedented polymorph of a TMDC 2D material. By mechanical exfoliation, we made thin flakes from a single crystal of 2Ha-type tantalum disulfide (TaS2), a metallic TMDC with a charge-density-wave (CDW) phase. Microbeam X-ray diffraction measurements and electrical transport measurements indicate that thin flakes possess a polymorph different from any one known in TaS2 bulk crystals. Moreover, the flakes with the unique polymorph displayed the dramatically enhanced CDW ordering temperature. The present results suggest the potential existence of diverse structural and electronic phases accessible only in 2D materials.

Characterization of locally strained Ge1−xSnx/Ge fine structures by synchrotron X-ray microdiffraction

We have investigated the formation of the locally strained Ge nanostructure with epitaxial Ge1−xSnx stressors and characterized the microscopic strain field in the Ge1−xSnx/Ge fine-heterostructures by synchrotron X-ray microdiffraction and finite element method (FEM) calculations. We achieved local epitaxial growth of Ge1−xSnx with Sn contents of 2.9% and 6.5%, sandwiching the 25 nm-wide Ge fine line structure. Microdiffraction measurements revealed that out-of-plane tensile strain induced in the Ge line effectively increased with decreasing Ge width and increasing Sn content of Ge1−xSnx stressors, which is in good agreement with FEM calculations. An out-of-plane tensile strain of 0.8% along the Ge[001] direction is induced in a 25 nm-wide Ge line, which corresponds to an in-plane uniaxial compressive strain of 1.4% in the Ge line sandwiched between Ge0.935Sn0.065 stressors.

Control of dislocation morphology and lattice distortion in Na-flux GaN crystals

The dislocation morphology and lattice distortion, including the tilting and twisting of lattice planes, at the Na-flux GaN/seed-GaN interface were investigated using transmission electron microscopy (TEM) and position-dependent nanobeam X-ray diffraction (nanoXRD). The results revealed that the dislocation morphology and lattice distortion in Na-flux GaN at the initial growth stage are strongly influenced by the seed-GaN surface morphology and the growth mode of Na-flux GaN. From the TEM results, one can observe that the formation of dislocation-related etch pits (DREPs) on the seed-GaN surface and the three-dimensional (3D) growth mode for Na-flux GaN give rise to the bending and lateral propagation of dislocations penetrating from the seed-GaN to the Na-flux GaN. This simultaneously results in homogenization of the GaN crystal domain structure as confirmed by nanoXRD. The mechanism responsible for the bending and lateral propagation of dislocations by the formation of DREPs and the 3D growth mode for t...